1. Field of the Invention
The present invention relates to a mechanism for varying performance of a set of intake valves or a set of exhaust valves in an engine. More particularly, the present invention pertains to a mechanism for varying valve timing and valve lift in an engine.
2. Description of the Related Art
A typical internal combustion engine includes a cylinder head provided with intake and exhaust valves. The intake valves selectively open and close intake passages connected to combustion chambers. Similarly, the exhaust valves selectively open and close exhaust passages connected to combustion chambers. In an engine having no mechanism for varying the performance of the valves, valve timing and valve lift of the intake and exhaust valves are constant at any given running state of the engine. The amount of intake air drawn into the combustion chambers and the amount of exhaust gas discharged from the combustion chambers directly correspond to the opening of a throttle valve and the engine speed.
In an engine equipped with a variable valve timing (VVT) mechanism, the valve timing of the intake valves, the exhaust valves, or both is varied. The VVT mechanism optimizes the valve timing (e.g. valve overlap) in accordance with the running state of the engine (engine load, engine speed and the like). This enhances the fuel economy and the power of the engine in wide range of different engine running states and reduces undesirable emissions.
Japanese Unexamined Patent Publication No. 6-234305 discloses an engine having a mechanism for varying the valve lift as well as a variable valve timing mechanism. The variable valve lift mechanism includes high speed cams used for a high engine speed and rocker arms that are actuated by the high speed cams. The mechanism also includes cams used for a low engine speed and rocker arms that are actuated by the low speed cams. Either the high speed cams and the high speed rocker arms or the low speed cams and the low speed rocker arms are selected in accordance with the running state of the engine. Switching the cams and the rocking arms changes the valve timing and valve lift. This further enhances the power and the fuel economy of the engine and reduces undesirable emissions.
However, the engine of this publication has the following problems:
(1) The variable valve timing mechanism and the variable valve lift mechanism both include an actuating system (a hydraulic circuit, a control circuit and the like). This complicates and enlarges the construction of the mechanisms. An engine having such mechanisms is larger and therefore requires more space in the engine compartment. PA1 (2) The mechanisms are independently controlled. It is thus difficult to simultaneously obtain an optimal valve overlap and an optimal valve lift corresponding to a running state of the engine. This is because different actuating speeds of the mechanisms result in variations of changing amount of valve timing and valve lift. PA1 (3) In the above described prior art variable valve lift mechanism, two pairs of cam and rocker arm are provided for a single valve. This complicates the construction of the mechanism.